The field of the invention relates to systems and methods for magnetic resonance imaging (“MRI”). More particularly, the present invention relates to systems and methods for adaptively gating of MRI acquisitions to allow for free-breathing cardiac MRI acquisitions.
MRI uses the nuclear magnetic resonance (“NMR”) phenomenon to produce images. When a substance such as human tissue is subjected to a uniform magnetic field, such as the so-called main magnetic field, B0, of an MRI system, the individual magnetic moments of the nuclei in the tissue attempt to align with this B0 field, but precess about it in random order at their characteristic Larmor frequency, ω. If the substance, or tissue, is subjected to a so-called excitation electromagnetic field, B1, that is in the plane transverse to the B0 field and that has a frequency near the Larmor frequency, the net aligned magnetic moment, referred to as longitudinal magnetization, may be rotated, or “tipped,” into the transverse plane to produce a net transverse magnetic moment, referred to as transverse magnetization. A signal is emitted by the excited nuclei or “spins,” after the excitation field, B1, is terminated, and this signal may be received and processed to form an image.
When utilizing these “MR” signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed for spatial encoding. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Patient motion has been a long-standing challenge to clinical MRI procedures. Patient motion can come in many forms, including bulk and fine motion or voluntary and involuntary motion. Substantial efforts have been made to control or overcome the errors and artifacts introduced by each patient motion.
In the case of involuntary or partially-involuntary motion, diaphragmatic and bellows navigators pulse sequences have been used to allow for the acquisition of free-breathing, three-dimensional cardiovascular MR images with reduced respiratory motion artifacts. The linear relationship between the respiratory motion of the right hemi-diaphragm (“RHD”) and the heart allows diaphragmatic navigators to track the RHD motion so that the respiratory motion of the heart can be indirectly corrected.
One method for mitigating patient motion artifacts is referred to as the “accept/reject algorithm.” In this method, the location of the RHD is measured during a preparatory phase to determine the location of the RHD at end-expiration. A small gating window, typically with a width of 5-7 mm, is then placed around the end-expiration position. Immediately before each acquisition of k-space lines, the RHD position is again measured. If the RHD position is within the gating window, the acquired k-space lines are accepted for image reconstruction; otherwise, those lines are rejected and reacquired until they are acquired within the gating window. This technique may be used with or without a slice tracking factor to acquire images with sub-millimeter accuracy. While the diaphragmatic navigator successfully suppresses the respiratory motion of the heart, this approach increases the duration of the MRI scan because the rejected k-space lines must be reacquired. Moreover, this approach results in an unpredictable scan acquisition time.
There have been several attempts to improve gating efficiency and reduce scan acquisition time without compromising image quality, including the use of k-space weighting, phase encode reordering, and diminishing variance algorithms. These algorithms reduce the acquisition time, but changes in the patient's breathing pattern can strongly reduce gating efficiency. To mitigate this problem and to maintain a high gating efficiency, an end-expiratory following technique has been proposed to track the position of the RHD at end-expiration and to update the location of the gating window. Although there is no image degradation using this technique compared to the fixed gating window position, the scan time and the range of diaphragm positions in the final image are still unpredictable and can be prolonged.
The other methods have also been proposed as alternatives to the accept/reject algorithm. These include phase ordering with automatic window selection (“PAWS”) and continuously adaptive window averaging (“CLAWS”), and were proposed to appropriately account for drifts and variations in breathing patterns. In PAWS and CLAWS, it is assumed that the data acquired at any RHD position may be used to reconstruct the final image; therefore, k-space lines are accepted and reordered using a predetermined algorithm to avoid duplications at different RHD positions. The scan is completed when all k-space lines are acquired within a gating window around an RHD position. These algorithms efficiently complete scans within a gating window in the presence of drifts and variations in breathing pattern, but their scan acquisition times are still long and unpredictable.
Therefore, it would be desirable to have a system and method for mitigating patient motion artifacts in MRI that overcome the limitations of existing methods. Notable limitations include the presence of drifts and variations in a patient's breathing pattern not being accounted for, thereby generating residual motion artifacts. Notable limitations also include unpredictable scan times resulting from no a priori information as to how many repetitions will be required to obtain a complete k-space data set.